Hey Irenduin,
Thanks for using Apple Support Communities. It's hard to tell from the image, but are you using a screen protector? If so, it could be the screen protector pulling away from the display. If you don't use a screen protector, try cleaning the display to see if it's a build-up of sorts. Here's a guide on cleaning to help: Cleaning your iPhone - Apple Support
If that doesn't help, we recommend having your iPhone inspected. You can locate an Apple Store or Apple Authorized Service Provider using this link: Find Locations - Apple
If you do take your iPhone in, be sure to prepare it for service, just in case. Here's how: Get your iPhone, iPad, or iPod touch ready for service - Apple Support
We hope this helps.
Take care!